The invention relates to a drive arrangement comprising a multi-phase linear or rotary motor of the synchronous type comprising a stator, a rotor or a translator, and at least two phase windings, an incremental rotor-position or translator-position measurement system which is responsive to detection pulses to increment or decrement, depending on the direction of movement of the rotor or the translator, a position-measurement value p, and a motor-energising system for generating in the phase windings energising currents whose values are periodic functions of the position-measurement value p, the periods of the functions corresponding to a rotor or translator displacement of 360 electrical degrees and the phase difference between the energising currents defined by the periodic functions being constant.
The invention further relates to a motor-energising system for use in the drive arrangement. Such an arrangement and system are known from U.S Pat. No. 4,390,827.
The known drive arrangement comprises a linear motor having a stator comprising a plurality of successive permanent magnets of alternate polarity and a translator having two phase windings, which translator is movable relative to the stator. The position of the translator relative to the stator is represented by the count of a digital counter in the incremental detection system. The count provides the address for two memories in which two periodic functions with a mutual phase difference of 90 degrees are stored in the form of tables. By means of multipliers a control signal, which is representative of the desired amplitude of the energising current, is multiplied by the function values appearing on the outputs of the memories. In such a drive arrangement the position of the translator magnet field generated by the phase windings relative to the stator magnet field is always dictated by the actual position of the translator. The functions are selected in such a way that the forces generated by the energising currents have maximum values which are independent of the translator position in the case of a constant value of the control signal. It is important that the position-measurement value always represents the actual translator position. In the case of a difference between the translator position as represented by the position-measurement value and the actual translator position forces smaller than said maxima will be generated.
When the drive arrangement is put into operation the position-measurement value must be brought in conformity with the actual rotor or translator position. However, in an incremental position-measurement system the actual translator position is then generally unknown. This disadvantage is frequently a reason to use an absolute position-measurement system, because in such a measurement system in the actual position is known immediately when the measurement system is switched on. However, in comparison with an incremental position-measurement system an absolute position-measurement system has the drawback of a very complex construction.
In another solution to mitigate said drawback of the incremental position-measurement system the rotor or translator is set to a known reference position before the drive arrangement is put into operation and subsequently the position-measurement value is set to a value in conformity with the known reference position.
The initial value can be set, for example, by energising the phase windings with energising currents corresponding to an arbitrary position-measurement value, causing the rotor or translator to assume a known preferred position. Subsequently, the initial value can be set by changing said arbitrary position-measurement value by a predetermined value. The initial value can also be set by operating the motor as a stepping motor or synchronous motor, to move the rotor or translator to the known reference position, for example by means of a mechanical stop, and subsequently bringing the initial value in conformity with this reference position.
The above methods of setting the initial values all have the disadvantage that during adjustment of the energisation of the motor the rotor or translator performs an uncontrolled movement. This is particularly problematic if the drive arrangement is utilized in a robot or other type of positioning system. In such positioning systems the positioning element is allowed to move only within a limited permissible operating range. However, as a result of the uncontrolled movements it is not likely that the positioning element exceeds the permissible operating range so that for example as a result of a collision between the positioning element and an object situated outside the permissible operating range, the object or the positioning element may be damaged.
Generally, linear motors are equipped with limit switches which are arranged at the stator ends and which are actuated by the translator. By means of these limit switches the energisation of the motor is interrupted as soon as the translator has reached one end of the stator. If the translator is situated in the proximity of a limit switch when the drive arrangement is put into operation an uncontrolled movement of the translator may result in the limit switch being actuated, causing the energisation to be interrupted. Such an undesired interruption can be remedied only by withdrawing the translator from the limit switch.